Communication devices are becoming increasingly mobile with the proliferation of cellular telephone networks. These cellular networks now allow individuals to make a telephone call using the present public switched telephone network (PSTN) from virtually any location under their respective umbrellas of service.
This relatively new mobility is not restricted to voice communication. Computers and other digital data communications devices are used in remote locations for specific applications which access the PSTN via a cellular network. The computer may access the cellular network via a mobile station modem. Such applications may include, for example, a mobile station modem application used in remote diagnostics in which a remote computer station accesses a central mainframe computer system for particular applications such as, for example, remote digital data acquisition or transmittal.
The cellular networks may be digital and/or analog. The earliest cellular networks were analog employing frequency modulation techniques as known by those skilled in the art. However, as time progressed, the capacity of such networks could no longer meet demand. Digital cellular networks were then adopted that provided for greater capacity than the analog networks.
Digital cellular networks generally employ either time division multiple access techniques (TDMA) or code division multiple access techniques (CDMA), both of which are well known to those skilled in the art. These techniques generally introduce significant delay in the transmission of the voice signals due to the modulation tasks performed by the voice encoders resident in the network and in the mobile units as known to those skilled in the art. Such a delay can result in a significant echo signal which is heard by the talker on the mobile unit. In order to eliminate the echo, voice echo cancelers are activated and brought on-line when necessary.
The digital cellular networks generally assume that a voice signal is to be transmitted. The digital modulation techniques employed are typically designed specifically for voice and not digital data signals. Consequently, digital data signals generally are disrupted when transmitted using TDMA or CDMA techniques. Many of these digital cellular networks allow one to revert back to an analog cellular communication link in those particular cases when digital cellular networks cannot be used.
In the situation where a digital cellular network has defaulted to analog cellular communication, it is sometimes the case that voice echo cancelers that are used to prevent voice echo in a digital cellular networks are activated and brought on-line, even in the case of an analog cellular network link. These voice echo cancelers may disrupt digital data communication if on-line during digital data transmission, as is known to those skilled in the art.
To explain further, cellular networks include several predefined areas, or cells, that border each other. Each cell has its own transmitter and receiver to communicate with mobile cellular telephones. Generally, these transmitters and receivers facilitate several concurrent channels of analog voice communication, each channel having its own frequency.
As a cellular telephone that is in analog communication with the transmitter and receiver moves from cell to cell, the cell transmitter and receiver will hand off the communication link with the cellular telephone to the transmitter and receiver associated with the adjacent cell into which the cellular telephone has traveled. Also, it is possible that a handoff from one frequency channel to another may occur within a single cell due to, for example, interference caused by various obstructions as the network searches for a channel free of interference.
In the case of digital data communication, voice echo cancelers are disabled when a telephone call facilitating digital data transmission is first established to prevent digital data disruption as known by those skilled in the art. Note that the signals or other means used to disable echo cancelers are only used at the beginning of a telephone call. This methodology is employed because it is traditionally assumed that the circuit by which the telephone call is first established will not change throughout the duration of the telephone call. Thus, once echo cancelers are disabled at the beginning, it is generally assumed that there is no further need to disable the echo cancelers.
However, in the case of analog cellular networks operating in conjunction with a digital cellular network, an echo canceler may be introduced into the circuit after call initiation. Specifically, when a handoff occurs, the digital data transmission will be established using a new channel which may be a different channel within the same cell, or a channel in a new cell. In this situation, the new channel used may incorporate a new echo canceler which will cut off digital data communication.
Thus, a heretofore unaddressed need exits in the industry for providing a system and method for disabling voice echo concelers after call startup.